In electrically controlled fuel or air/fuel mixture supply systems such as a fuel injection system and a electronically controlled carburetor system, various engine and vehicle operational parameters are utilized for the precise control of the air/fuel ratio of an air/fuel mixture supplied to an internal combustion engine. The precise control of the air/fuel ratio is required for obtaining clean exhaust gases, high engine operational performance and for the improvement of the fuel consumption characteristic.
As an example, in an electronic fuel injection system, the fuel flow rate is determined basically in accordance with the amount of intake air and the rotational speed of the crankshaft of the engine. However, various parameters such as engine coolant temperature, atmospheric air pressure, and intake air temperature, are used for the correction or modification of the fuel flow rate basically determined by the above two major parameters. Furthermore, in a closed loop air/fuel ratio control system, a gas sensor is used to detect the concentration of a component in the exhaust gases to perform a feedback control.
In the above parameters, the engine coolant temperature is one of the important parameters since this parameter is used to supply a rich air/fuel mixture to an internal combustion engine during warming up so as to render the engine rotation smooth and accelerate the warming up. Usually, the engine temperature is sensed by a temperature sensor such as a thermister, disposed in the water jacket in the engine casing to be exposed to the engine coolant.
A problem occurs when the engine temperature sensor is out of order. If the output signal of the temperature sensor does not change in accordance with the variation in engine coolant temperature, an undesirable phenomenon is apt to occur. For instance, a rich mixture is continuously supplied to the engine even after the engine is warmed up enough. In this case, the efficiency of the mechanism for the purification of the exhaust gases, such as a catalytic converter, and the fuel consumption characteristic may be deteriorated.